Radiometric dating assumptions




To explain those rules, I'll need to talk about some basic atomic physics. There are 90 naturally occurring chemical elements. Elements are identified by their atomic number , the number of protons in the atom's nucleus. All atoms except the simplest, hydrogen- 1, have nuclei made up of protons and neutrons. Hydrogen-1's nucleus consists of only a single proton. Protons and neutrons together are called nucleons , meaning particles that can appear in the atomic nucleus.

A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons. Since all atoms of the same element have the same number of protons, different nuclides of an element differ in the number of neutrons they contain. For example, hydrogen-1 and hydrogen-2 are both nuclides of the element hydrogen, but hydrogen-1's nucleus contains only a proton, while hydrogen-2's nucleus contains a proton and a neutron. Uranium contains 92 protons and neutrons, while uranium contains 92 protons and neutrons.

Many nuclides are stable -- they will always remain as they are unless some external force changes them. Some, however, are unstable -- given time, they will spontaneously undergo one of the several kinds of radioactive decay, changing in the process into another element.


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  • 2. Radiometric dating and testing for contamination and disturbances.

There are two common kinds of radioactive decay, alpha decay and beta decay. In alpha decay, the radioactive atom emits an alpha particle. An alpha particle contains two protons and two neutrons. After emission, it quickly picks up two electrons to balance the two protons, and becomes an electrically neutral helium-4 He4 atom. When a nuclide emits an alpha particle, its atomic number drops by 2, and its mass number number of nucleons drops by 4. Thus, an atom of U uranium, atomic number 92 emits an alpha particle and becomes an atom of Th thorium, atomic number A beta particle is an electron.

When an atom emits a beta particle, a neutron inside the nucleus is transformed to a proton. The mass number doesn't change, but the atomic number goes up by 1. Thus, an atom of carbon C14 , atomic number 6, emits a beta particle and becomes an atom of nitrogen N14 , atomic number 7. A third, very rare type of radioactive decay is called electron absorption.

In electron absorption, a proton absorbs an electron to become a neutron. In other words, electron absorption is the exact reverse of beta decay. So an atom of potassium K40 , atomic number 19 can absorb an electron to become an atom of argon Ar40 , atomic number The half-life of a radioactive nuclide is defined as the time it takes half of a sample of the element to decay. A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the nuclide.

Some nuclides have very long half-lives, measured in billions or even trillions of years. Others have extremely short half-lives, measured in tenths or hundredths of a second. The decay rate and therefore the half-life are fixed characteristics of a nuclide. Different nuclides of the same element can have substantially different half-lives. The half-life is a purely statistical measurement. A sample of U ten thousand years old will have precisely the same half-life as one ten billion years old.

Obviously, the major question here is "how much of the nuclide was originally present in our sample? Such cases are useless for radiometric dating. We must know the original quantity of the parent nuclide in order to date our sample radiometrically. Fortunately, there are cases where we can do that. This is the second axiom of radiometric dating. The third and final axiom is that when an atom undergoes radioactive decay, its internal structure and also its chemical behavior change.

Losing or gaining atomic number puts the atom in a different row of the periodic table, and elements in different rows behave in different ways. It may not form the same kinds of compounds. When the number of electrons change, the shell structure changes too. So when an atom decays and changes into an atom of a different element, its shell structure changes and it behaves in a different way chemically.

How do these axioms translate into useful science? This section describes several common methods of radiometric dating. To start, let's look at the one which almost everyone has heard of: The element carbon occurs naturally in three nuclides: C12, C13, and C The vast majority of carbon atoms, about About one atom in billion is C The remainder are C Of the three, C12 and C13 are stable.

C14 is radioactive, with a half-life of years. C14 is also formed continuously from N14 nitrogen in the upper reaches of the atmosphere. And since carbon is an essential element in living organisms, C14 appears in all terrestrial landbound living organisms in the same proportions it appears in the atmosphere.

Plants and protists get C14 from the environment. Animals and fungi get C14 from the plant or animal tissue they eat for food. When an organism dies, it stops taking in C If we measure how much C14 there currently is, we can tell how much there was when the organism died, and therefore how much has decayed.

When we know how much has decayed, we know how old the sample is. Many archaeological sites have been dated by applying radiocarbon dating to samples of bone, wood, or cloth found there. Radiocarbon dating depends on several assumptions. One is that the thing being dated is organic in origin.

Radiocarbon dating does not work on anything inorganic, like rocks or fossils. Only things that once were alive and now are dead: The second assumption is that the organism in question got its carbon from the atmosphere. A third is that the thing has remained closed to C14 since the organism from which it was created died. The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died.

That last one is more important than it sounds. When Professor William Libby developed the C14 dating system in , he assumed that the amount of C14 in the atmosphere was a constant. A long series of studies of C14 content produced an equally long series of corrective factors that must be taken into account when using C14 dating. So the dates derived from C14 decay had to be revised.

One reference on radiometric dating lists an entire array of corrective factors for the change in atmospheric C14 over time. C14 dating serves as both an illustration of how useful radiometric dating can be, and of the pitfalls that can be found in untested assumptions.

U and U are both nuclides of the element uranium. U is well known as the major fissionable nuclide of uranium. It has a half-life of roughly million years. U is more stable, with a half-life of 4. Th is the most common nuclide of the element thorium, and has a half-life of All three of these nuclides are the starting points for what are called radioactive series. A radioactive series is a sequence of nuclides that form one from another by radioactive decay. The series for U looks like this: A indicates alpha decay; B indicates beta decay.

We can calculate the half-lives of all of these elements. All the intermediate nuclides between U and Pb are highly unstable, with short half-lives. Then any excess of Pb must be the result of the decay of U When we know how much excess Pb there is, and we know the current quantity of U, we can calculate how long the U in our sample has been decaying, and therefore how long ago the rock formed. Th and U also give rise to radioactive series -- different series from that of U, containing different nuclides and ending in different nuclides of lead.

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Radiocarbon Dating and Bomb Carbon

Chemists can apply similar techniques to all three, resulting in three different dates for the same rock sample. Uranium and thorium have similar chemical behavior, so all three of these nuclides frequently occur in the same ores. If all three dates agree within the margin of error, the date can be accepted as confirmed beyond a reasonable doubt. Importantly, we can never prove that G actually is a constant, because doing that would require us to test G against every single piece of matter in the universe. This is where things get interesting and problematic for creationists.

Any high school level physics course will go over calculations that use G, and it is extremely important for astrophysics. Imagine for a moment that an astrophysicist derived an explanation for some phenomena, and the math for that explanation involved G. Therefore, via inductive logic, we must accept that it is constant until we have been shown a compelling reason to think that it is not constant.

1. How does radiometric dating work?

Even so, we have measured the rates of radiometric decay over and over again and they have always been constant. Therefore, via inductive logic, we must accept that they are constant until we have been shown a compelling reason to think that they are not constant. Similarly, we have repeatedly measured coral growth rates, and we know that even their fastest growth rate is nowhere near fast enough for them to have formed in only a few thousand years.

Also, note that the argument that creationists are making here is nothing more than an ad hoc fallacy. There is absolutely no reason to think that coral reefs grew faster in the past, or ice cores and varves formed multiple layers annually, or radioactive particles decayed faster, etc. First, realize that there are many different types of radiometric dating. Each method is specific to the type of rock that it can date, and which one you use depends on what type of material you are working with on a side note, you may see creationists claim that they have dated something that we know is recent, such as a rock from Mt St.

Helen, and the radiometric dating said it was old. These reports are generally a result of creationists using the wrong method for the rock in question. To illustrate how radiometric dating works, I am going to focus on one method uranium-lead dating , but all other types of radiometric dating follow the same general steps note: Uranium-lead dating is used on a type of rock known as a zircon.

Zircons are useful because when they form, the formation process incorporates uranium, but it strongly repels lead, which means that a newly formed zircon will never have any lead in it. Uranium exists in several isotopes same element, different numbers of neutrons , and the one we are interested in is U. A half-life is the amount of time that it takes for half the atoms to decay. For U, a half-life is roughly million years.

How do we know what the half-life is? After million years, it will have a 1: After another million years 1, million total , the ratio will be 1: After million more years 2, million total , the ratio will be 1: The ratios are the important things here, and they are why the amount of uranium in the original rock is irrelevant. We can take a zircon, measure the amount of U and the amount of Pb, and the ratio of those two chemicals will tell us how old the rock is.

Human Activities Affecting Carbon 14 Global Levels

For example, if the ratio is 1: In summary, radiometric dating is based on well tested, scientific results, not assumptions. We know that there was no lead in zircons to begin with, because zircons strongly repel lead when they are forming. Finally, we know the rate at which uranium decays into lead because we have repeatedly measured it, and it has always been the same. You understate the case for radiometric dating. Isochron methods, using a non-radiogenic isotopes to tell us the amount of daughter present to start with, avoids assumptions about initial amounts.

And the constancy of decay rates is not merely an observation made over the past century or so but confirmed by observations on distant supernovas that we are observing them at times ling past.

What are the assumptions used with radiometric dating? - Returning to Genesis

Moreover, these rates are consequences of such fundamental physical laws that we know they cannot have changed. For if those laws had been different, the whole of physical science would have been different, and we would not have had rocks of recognisable chemistry being laid down in the first place. Thank you for your comment. In the future I may write a more detailed post about the physics and math behind it, but in my many discussions with creationists, I have generally found that it is best to just stick to some basic points that are easy to grasp and avoid overloading them with too many facts.

And yet I was responding the arguments that creationists actually make, and that you will find spelt out in any creationist text. It is always a difficult judgement call; to what extent should we simply ignore the details of creationist arguments, and to what extent should we explicitly rebut them. The former risks giving a free pass to fallacies, while the latter risks spreading the creationist meme. Could these rates be affected by forces such as temperature, magnetic fields, or quantum vacuum fluctuations?

There is a considerable amount of literature on the topic of external factors affecting decay rates, and occasionally someone reports an anomalous result, but the overwhelming consensus is that they are not affected by things like temperature many of the anomalies are likely the result of user error.

Regarding changing radioactive decay rates; some rates do depend, in known and well understood ways, on the charge of the decaying atom, but the effects are minor except under conditions such as those inside stars. Geology, radiology, astronomy and biology all point to pretty consistent date ranges, and none of them can support anything remotely close to a literalist interpretation of the Bible.


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  8. It is very strange to encounter someone still proselytizing it. There is no sense in which you can go from a series of observations to a general law. Your example of gravity is quite revealing. G is a measured quantity. And despite the powers of induction, we now know there is no force of gravity.

    What are the assumptions used with radiometric dating?

    It is merely a form of probabilistic argument. All scientific theories and laws are arrived at by inductive logic. That is inherent in their nature. For example, cell theory states that all living things are made of cells. If the samples have been undisturbed closed systems since formation, the data will fall on the same line the isochron from which the diagram is named. The slope of this line is a function of the age of the rock. If the rock is older, the slope is higher. The reason scientists normalize with another stable isotope of the same element as the daughter is because most chemical or physical processes that occurs normally in nature does not differentiate between different isotopes of the same element when the difference in mass is as small as it is between isotopes of the same element that is used in radiometric dating.

    This means that the while different rocks contain different absolute amounts of the two isotopes, the ratio is same. At the time of formation for a rock, the isotopes for an element are homogenized and so the composition of a certain isotope is the same in all the minerals in the rock. But what happens when the rocks have been disturbed? If so, the data will not fall on an isochron line, but will be all over the place. This tells scientists that the sample has been disturbed and cannot be dated with this particular method.

    So far from rejecting samples because they do not fit a preconceived notion of what the age should be, scientists reject samples because there is ample evidence that it has been disturbed: Scientists do not assume that rocks have been closed systems; it is a well-supported conclusion from experiments. But what about assuming that initial amounts are known? A second property of isochron diagrams is that it actually gives the initial amount of daughter isotope as a result of the method.

    It is just the y-intercept of the isochron line. The initial conditions are just read off the graph; it is not just assumed. In a last ditch effort, young earth creationists exclaim that scientists just assume, without warrant, that decay rate are constant. However, this is not the case.

    Decay rates have been shown to be constant, despite very high pressure and temperature. Furthermore, by studying supernovas far away, scientist have determined that decay rates have been constant in the ancient past as well. Not only that, different radioactive isotopes decay differently and it is enormously improbable that a postulated difference in decay rates would affect all of them in the same way, yet as we have seen, different radiometric dating methods converge on the same date within margins of error. Fourthly, decay rates can be predicted from first principles of physics.

    Any change would have to correspond to changes in basic physical constants.

    Part I - What Is Radioactive Dating & Its Assumptions?

    Any such change would affect different forms of decay differently, yet this has not been observed. As a final blow to the already nailed shut coffin of young earth creationism, had decay rates been high enough to be consistent with a young earth, the heat alone would have melt the earth. Scientists do not assume that rocks have been closed systems, but they test for it. If all the data points fall on the isochron line, it has been a closed system; it it scatters, it has not and that rock is not used for dating with that method.

    Scientists also do not assumed that initial conditions are known; this is just read off the graph at the y-intercept. Finally, by studying supernovas, scientists know that decay rates have been constant in the past. Portrait of a Planet. Constancy of Radioactive Decay Rates. Geochronometry and closed systems.

    Geochronology and initial conditions. Refer to the right hand side of the table in the website address, http: A list of percentage remaining that corresponds to the number of the relative half-lives elapsed are presented as follows:

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